1. Field of the Invention
This invention relates to stabilized 2-cyanoacrylate adhesive compositions. More particularly, this invention is directed to a novel class of anionic polymerization inhibitors for 2-cyanoacrylate adhesive compositions.
2. Description of the Prior Art
Adhesive compositions based on 2-cyanoacrylate esters belong to a class of adhesives known as reactive liquid adhesives. 2-Cyanoacrylate adhesives are single-part, low-viscosity adhesives which are characterized by features such as (1) their ability to polymerize at room temperature without the use of an added catalyst when pressed between two substrates, (2) their rapid rate of cure, and (3) the strength of the bonds produced with a wide variety of substrates. Conventional adhesives, on the other hand, cure, for example, upon application of heat and pressure, addition of catalyst, or evaporation of a solvent. A general review of 2-cyanoacrylate adhesives can be found in I. Skeist's "Handbook of Adhesives", New York: Reinhold Publishing Corporation, 1962, Chapter 31, p. 409-414.
Application of the 2-cyanoacrylate adhesive merely involves spreading a small sample thereof in a thin film between two substrates, pressing the substrates together, and allowing the resultant bond to cure. The adhesive develops sufficient strength after a short period of time to hold the substrates together until the adhesive completely polymerizes and builds up to its maximum bonding strength.
Initiation of polymerization (cure) is generally believed to proceed through an anionic mechanism; the 2-cyanoacrylate adhesives have such a great tendency to polymerize the water itself is a sufficiently active initiator. Hence, when the adhesive is applied to a substrate and thereby exposed to atmospheric and surface moisture, cure normally begins within a relatively short period of time, generally less than one minute, and on many surfaces within a matter of a few seconds. The rapid cure rate of the 2-cyanoacrylate adhesives is particularly advantageous in production line applications.
Due to their tendency to polymerize, 2-cyanoacrylate adhesive compositions normally contain one or more stabilizers. To prevent anionic polymerization an inhibitor such as an acidic gas or a protonic or Lewis acid is normally added to the composition. Examples of acidic gases used for this purpose include sulfur dioxide, nitric oxide, carbon dioxide, hydrogen fluoride, etc. Known protonic acids include mineral acids such as hydrochloric or sulfuric acid, sulfonic acids, and carboxylic acids such as acetic, trichloroacetic, acrylic, methacrylic, and itaconic acid. Examples of anhydrides which are known anionic polymerization inhibitors are carboxylic acid anhydrides such as itaconic and maleic anhydride; phosphoric anhydrides such as phosphorus pentoxide; antimony pentoxide; sultones; acid chlorides; and the like. Anionic polymerization inhibitors which are Lewis acids include stannic chloride, ferric chloride, and boron trifluoride and its etherate complexes. Typical patents disclosing these and other stabilizers are U.S. Pat. Nos. 2,756,251; 2,912,454; 2,926,188; 3,728,375; and 3,993,678; Jap. Pat. Publication No. 49-31619; and Ger. Offen. No. 2,307,834.
Free radical polymerization is generally inhibited in the 2-cyanoacrylate adhesives, if necessary, by adding phenolic-type compounds such as hydroquinone, pyrogallol, or t-butyl catechol thereto.
The acids used to stabilize the adhesive against anionic polymerization must be used with great discretion. Very strong acids, if added in large amounts, can lead to overstabilization of the adhesive; however, weak acids are generally not so effective as the stronger acids in achieving stabilization. In addition, all of the acids which act as anionic polymerization inhibitors exert a retarding effect on the cure rate of the adhesive, to a greater or lesser degree, depending on the specific acid used. Carboxylic acids, for example, generally retard the cure rate to a large extent.
Accordingly, it is an object of the present invention to provide a class of effective anionic polymerization inhibitors for 2-cyanoacrylate adhesive compositions which do not significantly retard the cure rate of the adhesive.